1. Field of the Invention
The present invention relates to a fluid-circulation heat exchanger, in particular for an electron tube. Some electron tubes have high thermal stresses. These stresses are associated with two types of factors: either the intended average power performances are at the limit of known cooling systems, or the allowed size does not make it possible to develop appropriate heat exchangers.
2. Discussion of the Background
In gridded microwave tubes, it is the anode which requires efficient cooling; in linear-beam tubes it is, in particular, the collector which must be cooled, and also the drift tubes and the cavity ports in klystrons or travelling-wave tubes with coupled cavities. Crossed-field tubes must also be cooled at their anode. As regards fast-wave tubes such as gyrotrons, it is the cavity and/or the collector which requires effective cooling.
Those heat exchangers with the best performance which are currently used operate with a fluid.
In some heat exchangers, the part to be cooled is immersed in the fluid, which is often water. This type of exchanger is limited in terms of heat-flux density because a vapour film causes stagnation at the surface of the part to be cooled. This is the burn-out phenomenon. This vapour has low thermal conductivity. The part consequently heats up enormously and the heat exchanger no longer fulfils its function. This temperature rise may lead to destruction of the part to be cooled.
One known solution consists in using a forced fluid stream at high flow rate and high pressure around the part, but this requires voluminous and expensive equipment with a compressor whose energy consumption is far from negligible. These heat exchangers nevertheless have high performance.
Another solution consists in using heat exchangers of the VAPOTRON or derived type. The term VAPOTRON is a registered trademark belongs to the company Thomson SA. They use vaporization of the liquid in contact with a hot wall, this wall being provided with reliefs which promote pulsed expulsion of the vapour, which alternately provides access of the liquid against the wall to be cooled. Liquid circulation compatible with the mains distribution is sufficient.
These heat exchangers have only made it possible to achieve continuous heat-flux densities of 1 kW/cm.sup.2 and, exceptionally, 2 kW/cm.sup.2. These performances are poorer than those of forced-circulation exchangers, but they do not require an installation delivering a forced steam of fluid. However, these heat exchangers are relatively heavy and expensive because of the bulky relief wall.
Document JP-A-53 91 164 discloses a porous body formed by thin tubes grouped in a bunch in a pipe and assembled together with the aid of molten metal.